Organic electroluminescent devices comprise a layer of organic electroluminescent material positioned between two electrodes. Application of a current between the two electrodes causes the organic material to electroluminescence. The phenomenon of organic electroluminescence has many applications in the fields of displays and lighting. In the late 1980's efficient organic electroluminescence was observed in conjugated polymers, as described in WO90/13148, and in complexes of aluminum 8-hydroxyquinolate, as described in Tang et al Applied Physics Letters 51, 913, (1987). The electroluminescence in such systems is termed fluorescence and is produced by radiative emission from the singlet excited states produced by the electrical excitation of the polymers or molecules. Much research over the last decade has been directed at the investigation of such fluorescent systems.
In the 1990s the efficient emission of light from the triplet excited states of electrically excited molecules was observed, Baldo et al Applied Physics Letters 75, 4, (1999). This electroluminescent system comprised a green light emitting cyclometallated iridium phenylpyridine complex and showed a higher efficiency than had previously been observed in fluorescent systems. This phenomenon, known as phosphorescence, has been widely investigated. US2002/0134984 discloses a series of iridium complexes in which iridium is coordinated to a bidentate ligand via two nitrogen atoms, such as compound 1 below. US2001/0019782 discloses a series of iridium complexes in which iridium is coordinated to a bidentate ligand, the ligand comprising two aryl moieties bonded by a C—N bond, such as compound 2 below. US2002/0055014 discloses a series of iridium complexes in which iridium is coordinated to a ligand comprising a phenylazole derivative, such as compound 3 below. WO02/15645 discloses the blue phosphorescent complex 4, known as Firpic, shown below.

The cyclometallated phosphorescent complexes referred to above have been designed to be volatile to allow their deposition by vacuum evaporation. Vacuum evaporation is a technique commonly used to deposit layers of low molecular weight materials in the manufacture of electroluminescent devices. Other research has focussed on the incorporation of phosphorescent emitters into polymers, as disclosed in WO02/068435 and EP1245659. The incorporation of phosphorescent emitters into polymers allows the phosphorescent systems to be deposited using solution processing techniques such as ink-jet printing and screen printing. The advantage of using solution processing to produce electroluminescent displays has been widely recognised in the field, as disclosed for example in EP 0 880 303.
An alternative approach to provide solution processable phosphorescent materials has been to incorporate the phosphorescent emitters into dendrimers, as disclosed in WO02/066552. Dendrimers are highly branched macromolecules in which branched molecular sub-units, known as dendrons or dendrites, are attached to a core. The dendrimers disclosed in WO02/066552 comprise a cyclometallated phosphorescent core and a series of organic dendrons. The properties of such dendrimers make them ideal for solution processing. Further classes of electroluminescent dendrimers have been developed including dendrimers comprising dendrons based on aryl-aryl moieties, as disclosed in WO02/067343, and asymmetric dendrimers as disclosed in WO02/066575.
Much of the development of organic light emitting devices is aimed at the exploitation of these devices in display applications such as mobile phones and large area displays. Full colour displays require light emitting materials which emit light in the red, green and blue regions of the electromagnetic spectrum. Fluorescent organic materials capable of emitting red, green and blue light have been developed.
Phosphorescent materials emitting red and green light have been developed but there are relatively few examples of phosphorescent materials capable of emitting blue light. Although the above mentioned iridium complex Firpic emits blue light this is of a light blue colour rather than the deeper blue required for full colour displays.
As is clear from the above brief survey of recent research in the field there is a need for the development of phosphorescent compounds having a high efficiency and available in a range of colours suitable for use in display applications. In particular there is a need for a range of high efficiency phosphorescent complexes emitting light in the blue region of the electromagnetic spectrum. Further there is a need for a range of solution processable phosphorescent light emitting compounds.